Light guide apparatus and fabrication method thereof

ABSTRACT

A light guide apparatus that can redirect light impinging on the apparatus over a wide range of incident angles and can concentrate light without using a tracking system and methods for fabrication.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of co-pending U.S. patent applicationSer. No. 14/148,388, filed Jan. 6, 2014, entitled LIGHT GUIDE APPARATUSAND FABRICATION METHOD THEREOF, which in turn claims priority to andbenefit of U.S. Provisional Application No. 61/749,168, filed Jan. 4,2013, the contents of which are incorporated by reference herein intheir entirety for all purposes.

BACKGROUND

The present teachings relate to a light guide apparatus and afabrication method thereof. More particularly, the present teachingsrelate to a light guide apparatus for collecting light and deliveringthe collected light, and a fabrication method thereof.

Light guide apparatuses, such as light pipes and optical fibers, havebeen used to redirect the propagation of light beams using the principleof wave-guiding. Light pipes need to have the light pumped into themfrom the end of the pipe or fiber with the condition that angle of lightfalls within the acceptance cone of the fiber to be guided therein.Light guide apparatuses, such as solar concentrators, have also beenused to focus light in a small area. However, solar concentratorsrequire a tracking system, because they only work at certain fixed angleof the sun relative to the solar concentrators. Moreover, many of theselight guide apparatuses are rather bulky. Accordingly, there is a needto develop a new light guide apparatus that can concentrate lightwithout using a tracking system. There is also a need for a new lightguide apparatus that can redirect light impinging on the apparatus overa wide range of incident angles.

SUMMARY

A light guide apparatus that can redirect light impinging on theapparatus over a wide range of incident angles and can concentrate lightwithout using a tracking system and methods for fabrication aredisclosed hereinbelow.

In one aspect, the present disclosure provides a light guide apparatushaving a core that defines a longitudinal axis. The core includes afirst optically transparent section comprising a first optical mediumhaving a first index of refraction; and a second optically transparentsection comprising a second optical medium having a second index ofrefraction, an interface between the first optically transparent sectionand the second optically transparent section defining a shape. Theshape, the first index of refraction, and the second index of refractionare configured such that light entering the core is deflected at theinterface at an angle such that, when the light impinges on acore-cladding interface, the light impinges on the core claddinginterface at an angle at least equal to a critical angle for totalinternal reflection.

In one embodiment, the shape comprises a first frustum of a first halfangle, wherein a central axis of the first conical frustum substantiallycoincides with the longitudinal axis of the core.

In one embodiment, the core further comprises a third opticallytransparent section comprising a third optical medium, the thirdoptically transparent section interfacing with the second opticallytransparent section to define a central cylinder. The third opticalmedium may be air or vacuum. The second optical medium may be same asthe third optical medium.

In one embodiment, the shape further comprises a second conical frustumof a second half angle, a central axis of the second conical frustumsubstantially coinciding with the longitudinal axis of the core. Thesecond half angle is greater than the first half angle. A bottom basecircumference of the first conical frustum coincides with a bottom basecircumference of the second conical frustum. A top base circumference ofthe first and second conical frustums substantially coincides with acurvilinear side surface of the central cylinder. A bottom basecircumference of the first conical frustum substantially coincides witha bottom base circumference of the second conical frustum.

In one embodiment, the first half angle ranges from about 0.05 degreesto about 50 degrees, and the second half angle ranges from about 2degrees to about 80 degrees.

In one embodiment, the second frustum comprises a semicircular conicalfrustum, and the first conical frustum comprises a semicircular conicalfrustum.

In one embodiment, the first refractive index of the first opticalmedium ranges from about 1.4 to about 2.2, and the second refractiveindex of the second optical medium ranges from about 1.3 to about 2.0.

In one embodiment, the shape comprises a plurality of first conicalfrustums of the first half angle and a plurality of second conicalfrustums of the second half angle, wherein central axes of the first andsecond conical frustums substantially coincide with the longitudinalaxis of the core. One of the first conical frustums comprises a top basecircumference that coincides with a top base circumference of aneighboring one of the second conical frustums.

In one aspect, the present disclosure provides a light guide apparatuscomprising a core defining a longitudinal axis; and a cladding layer onthe core. The cladding layer comprises a first optical medium having afirst index of refraction and an inclusion structure embedded in theoptical medium. The inclusion structure comprises a second opticalmedium having a second index of refraction. The inclusion structuredefines an interface between the first optical medium and the secondoptical medium. The inclusion structure, the first index of refraction,and the second index of refraction are configured such that lightincident on the interface is totally internally reflected and propagatesat a predetermined grazing angle with respect to the longitudinal axis.The light is incident on the cladding layering a predetermined range ofangles from a normal direction substantially perpendicular to thelongitudinal axis.

In one embodiment, the interface has one of a conic shape, a semi-conicshape, a parabolic conic shape, and an ellipsoidal shape. Surfaces ofthe inclusion structure may be textured, and the second optical mediummay be air. The inclusion structure may have a semi-conic shape. Thecore may have a semi-cylindrical shape, and the core may be tapered.

In one embodiment, a cross section of the cladding layer has an outercircumference of a shape selected from the group consisting of a circle,an N-sided polygon, an ellipse, a semicircle, and a bounded shape of twocircular arcs, wherein N is a natural number ranging from 3 to 100. Inone embodiment, the first refractive index of the first optical mediumof the cladding layer ranges from about 1.3 to about 1.8.

In one embodiment, the core comprises at least an optically transparentmedium, and a refractive index of the optically transparent medium ofthe cylindrical core is greater than that of the first optical medium ofthe cladding layer.

In one embodiment, the core comprises a first optically transparentsection comprising a third optical medium having a third index ofrefraction; and a second optically transparent section comprising afourth optical medium having a fourth index of refraction; an interfacebetween the third optically transparent section and the optically fourthtransparent section defining a shape; the shape, the third index ofrefraction and the fourth index of refraction being configured such thatlight entering the core is deflected at the interface at an angle suchthat, when the light impinges on a core-cladding interface, the lightimpinges on the core cladding interface at an angle at least equal to acritical angle for total internal reflection.

In one embodiment, the shape comprises a first conical frustum of afirst half angle, wherein a central axis of the first conical frustumsubstantially coincides with the longitudinal axis of the cylindricalcore. The cylindrical core further comprises a third transparent sectioncomprising a third optical medium, the third transparent sectioninterfacing with the second transparent section to define a centralcylinder. The shape further comprises a second conical frustum of asecond half angle, a central axis of the second conical frustumsubstantially coinciding with the longitudinal axis of the cylindricalcore.

In one aspect, the present disclosure provides a light guide apparatuscomprising a core defining a longitudinal axis, a cladding layer on thecore, and a super-cladding layer on the cladding layer. The claddinglayer is configured to deflect light incident on the cladding layer, thelight being incident on the cladding layer in a predetermined range ofangles from a normal direction substantially perpendicular to thelongitudinal axis. The light is deflected to a direction that forms agrazing angle with respect to the longitudinal axis. The super-claddinglayer comprises a first optically transparent medium that receivesincident light. A second optically transparent medium interfaces withthe first optically transparent medium to define a heterogeneousinterface. The heterogeneous interface comprises a plurality of bi-conicshapes.

In one aspect, the present disclosure provides a light guide apparatuscomprising a super-cladding layer, wherein the super-cladding layercomprises a first optically transparent medium that receives incidentlight; and a second optically transparent medium interfacing with thefirst optically transparent medium to define a heterogeneous interface,wherein the heterogeneous interface comprises a plurality of shapes;each shape from the plurality of shapes configured to deflect a lightbeam, incident on the super-cladding layer in a first predeterminedrange of angles from a normal direction substantially perpendicular to alongitudinal axis, into a second predetermined range of angles.

In one embodiment, the light guide apparatus further comprises a coredefining the longitudinal axis; a cladding layer disposed on the core,wherein the cladding layer is configured to deflect light incident onthe cladding layer, the light being incident in said secondpredetermined range of angles from a normal direction substantiallyperpendicular to the longitudinal axis; the light being deflected to adirection that forms a grazing angle with respect to the longitudinalaxis. The plurality of shapes comprises a plurality of bi-conic shapes;and wherein an angle subtended by surfaces of two neighboring bi-conicshapes ranges from about 2 degrees to about 30 degrees.

In one embodiment, a refractive index of the first optically transparentmedium ranges from about 1.3 to about 2.2, and a refractive index of thesecond optically transparent medium ranges from about 1.35 to about 2.0.A refractive index difference of the first and second opticallytransparent media may range from about 0.02 to about 0.30. Thesuper-cladding layer is configured to convert a light beam having anincident angle within about ±30 degrees with respect to the normaldirection to a light beam having an angle within about ±5 degrees orless with respect to the normal direction.

In one embodiment, the core comprises a third optically transparentsection comprising a first optical medium with a first index ofrefraction; and a fourth optically transparent section comprising asecond optical medium with a second index of refraction; an interfacebetween the third optically transparent section and the fourth opticallytransparent section defining a shape; the shape, the first index ofrefraction and the second index of refraction being configured such thatlight entering the core is deflected at the interface at an angle suchthat, when the light impinges on a core-cladding interface, the lightimpinges on the core cladding interface at an angle at least equal to acritical angle for total internal reflection.

In one embodiment, the shape comprises a first conical frustum of afirst half angle, wherein a central axis of the first conical frustumsubstantially coincides with the longitudinal axis of the core.

In one embodiment, the cladding layer comprises a third opticallytransparent section having a first index of refraction and an inclusionstructure embedded in the third optical medium; the inclusion structurecomprising a fourth optical medium having a second index of refraction;the inclusion structure defining an interface between the third opticalmedium and the fourth optical medium; the inclusion structure, the firstindex of refraction and the second index of refraction configured suchthat light incident on the interface is totally internally reflected andpropagates at a predetermined grazing angle with respect to thelongitudinal axis; the light being incident in a predetermined range ofangles from a normal direction substantially perpendicular to thelongitudinal axis. The grazing angle may range from about 0.5 degrees toabout 35 degrees.

In one aspect, the present disclosure provides a solar panel comprisinga light guide apparatus for receiving sun light, the light guideapparatus comprising a plurality of parallel light pipes, and aphotovoltaic cell optically coupled to an end of the parallel lightpipes. Each light pipe comprises a core defining a longitudinal axis anda normal direction substantially perpendicular to the longitudinal axis;a cladding layer on the core, wherein the cladding layer is configuredto convert sun light from a first predetermined range of angles (about±5 degrees in an exemplary embodiment) with respect to the normaldirection to a direction that forms a grazing angle with respect to thelongitudinal axis; and a collimating layer on the cladding layer,wherein the collimating layer is configured to convert sun light from anincident angle within a second predetermined range of angles (in anexemplary embodiment about ±30 degrees) with respect to the normaldirection to an angle within the first predetermined range of angles (inan exemplary embodiment, about ±5 degrees) with respect to the normaldirection.

In one embodiment, the solar panel further comprises a reflector underthe light guide apparatus for reflecting escaped light back to the lightguide apparatus.

In one aspect, the present disclosure provides a method of fabricating alight guide assembly. The method comprises forming a number ofprotrusions on a surface of a first optically transparent material;forming a number of indentations on a surface of a second opticallytransparent material; each indentation from the number of indentationsbeing configured so that a shape of said each indentation is similar toa shape of each protrusion from the number of protrusions and dimensionsof said each indentation are bigger than dimensions of said eachprotrusion; and assembling said surface of the first opticallytransparent material over said surface of the second opticallytransparent material such a space is disposed between surfaces and alocation of each protrusion corresponds to a location of eachindentation, forming a number of inclusions. In one embodiment, air isdisposed in the space between the surfaces. In one embodiment, themethod further comprises depositing a layer of a third opticallytransparent material of a substantially constant thickness over saidsurface of the first optically transparent material; said substantiallyconstant sickness configured such that a shape of said surface of thefirst optically transparent material, after deposition is substantiallycongruent with a shape of said surface of the second opticallytransparent material; wherein after assembling the third opticallytransparent material is disposed in the space between protrusions andindentations.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is to be read in conjunction with theaccompanying drawings, in which:

FIGS. 1(a) through 1(d) illustrate a light guide apparatus having a coreand a cladding layer, in accordance with various embodiments of thepresent disclosure;

FIGS. 2(a) through 2(d) illustrate a light guide apparatus having acore, in accordance with various embodiments of the present disclosure;

FIG. 3 illustrates a light guide apparatus having a core, a claddinglayer, and a super-cladding layer, in accordance with one embodiment ofthe present disclosure;

FIGS. 4(a) and 4(b) illustrate ray tracing diagrams for a light guideapparatus having a core and a cladding layer, in accordance with oneembodiment of the present disclosure;

FIG. 5 illustrates a ray tracing diagram for a light guide apparatushaving a core, a cladding layer, and a super-cladding layer, inaccordance with one embodiment of the present disclosure;

FIGS. 6(a) through 6(c) illustrate ray tracing diagrams for a lightguide apparatus having a core in accordance with various embodiments ofthe present disclosure:

FIG. 7 illustrates a perspective view of a solar panel including anarray of light guide apparatuses, in accordance with one embodiment ofthe present disclosure;

FIGS. 8(a) through 8(g) illustrate various views of systems including anarray of light guide apparatuses, in accordance with various embodimentsof the present disclosure;

FIGS. 9(a) through 9(f) illustrate a planar light guide apparatuses, inaccordance with various embodiments of the present disclosure:

FIGS. 10(a) through 10(c) illustrate strategies for materials selectionin accordance with various embodiments of the present disclosure;

FIGS. 11(a) and 11(b) illustrate a method for fabricating a light guideapparatus, in accordance with one embodiment of the present disclosure;

FIG. 11(c) is a flow chart of the method of one embodiment of thepresent disclosure;

FIG. 12(a) illustrates a smart window using a light guide apparatus, inaccordance with one embodiment of the present disclosure;

FIG. 12(b) illustrates a lighting device using a light guide apparatusin accordance with one embodiment of the present disclosure;

FIG. 12(c) illustrates optical collectors for indoor lighting using alight guide apparatus in accordance with one embodiment of the presentdisclosure

FIG. 12(d) illustrates solar thermal device using a light guideapparatus in accordance with one embodiment of the present disclosure;

FIG. 12(e) illustrates optical laminates on photovoltaic devices andmodules using a light guide apparatus in accordance with one embodimentof the present disclosure;

FIG. 12(f) illustrates light trapping optics for luminescentconcentrators using a light guide apparatus in accordance with oneembodiment of the present disclosure; and

FIG. 12(g) illustrates a device for optical pumping of lasers using alight guide apparatus in accordance with one embodiment of the presentdisclosure.

DETAILED DESCRIPTION

Methods and systems for collecting light into a light guide apparatus ofthese teachings as the light impinges on the apparatus over a wide rangeof angles are disclosed herein below.

In one instance, described herein includes an apparatus and a method topump light into the light pipe from sides of the light pipe along thelength of the pipe so that, for a uniform illumination over the pipe,light ends up accumulating inside the core of the pipe while travelingalong the length of the pipe. This can be used to concentrate the lightinto core.

The following detailed description is of the best currently contemplatedmodes of carrying out these teachings. The description is not to betaken in a limiting sense, but is made merely for the purpose ofillustrating the general principles of these teachings, since the scopeof these teachings is best defined by the appended claims. Although theteachings have been described with respect to various embodiments, itshould be realized these teachings are also capable of a wide variety offurther and other embodiments within the spirit and scope of theappended claims.

As used herein, the singular forms “a,” “an,” and “the” include theplural reference unless the context clearly dictates otherwise.

Except where otherwise indicated, all numbers expressing quantities ofingredients, reaction conditions, and so forth used in the specificationand claims are to be understood as being modified in all instances bythe term “about.” Further, any quantity modified by the term “about” orthe like should be understood as encompassing a range of ±10% of thatquantity.

“Light,” as used herein refers to electromagnetic radiation and is notlimited to only the visible range of wavelengths.

The terms “light pipe,” “optical fiber,” and “optical pipe” are usedhereinbelow to describe the light guide apparatus of the presentdisclosure and are used interchangeably herein. The term optical pipeshould be taken to limit the embodiment to a particular geometry.

In one embodiment of the system of these teachings, the light isconcentrated into a compact design of the light pipe by accumulatinglight into the cores of an array of light pipes at wide angles ofincidence. This eliminates the need for solar tracking systems becausethe optics approach of these teachings can address the variation ofangles of incidence during the day as well as during the year. Such sidepumped optical pipes and planar concentrators that use the designelements of these teachings can be used for many applications such assolar panels, power producing smart windows, indoor lighting, solarthermal, etc., as described herein below.

In one or more embodiments, the light guide apparatus of these teachingsincludes a core defining a longitudinal axis, a cladding layer on thecore, the cladding layer having a first optical medium having a firstindex of refraction and an inclusion structure embedded in the opticalmedium, the inclusion structure including a second optical medium havinga second index of refraction, the inclusion structure defining aninterface between the first optical medium and the second opticalmedium, the inclusion structure, the first index of refraction and thesecond index of refraction configured such that light incident on theinterface is totally internally reflected and propagates at apredetermined grazing angle with respect to the longitudinal axis; thelight being incident in a predetermined range of angles from a normaldirection substantially perpendicular to the longitudinal axis.

In one instance, the shape of the interface is one of a conic shape, asemi-conic shape, a parabolic conic shape, and an ellipsoidal shape.

In another instance, surfaces of the inclusion structure are textured.

In yet another instance, the second optical medium is air, theseteachings not being limited to only that instance.

Although in the embodiments shown below the cladding and the core areshown as cylinders, a number of other geometries are within the scope ofthese teachings. For instance, the cladding and the core can be one of acircle, an N-sided polygon, an ellipse, a semicircle, or a bounded shapeof two circular arcs. In one embodiment, N is a natural number rangingfrom 3 to 100.

In still another instance, the inclusion structure has a semi-conicshape, and the core has a semi-cylindrical shape. In one embodiment, thefirst refractive index of the first optical medium of the cladding layerranges from about 1.3 to about 2.2.

FIGS. 1(a) through 1(d) illustrate a light guide apparatus having a core108 and a cladding layer 104, in accordance with various embodiments ofthe present disclosure. As shown in FIGS. 1(a) through 1(d), the lightguide apparatus 102 (or optical pipe 102 or fiber 102) comprises acladding layer 104 and a core 108. The general approach is to take lightincident on the outer surface of the optical pipe 102 and convert thelight to a grazing incidence with respect to the longitudinal axis 110of the optical pipe 102. This grazing incident light then enter the core108 of the fiber 102 which includes optical elements that trap andpropagate the light along the length of the fiber 102 inside the core108.

As shown in FIGS. 1(a) through 1(d), the cladding layer 104 includes amonolithic optically transparent medium with air inclusion 106 in theshape of a cone. The refractive index of this medium is between 1.3 and1.6. The half angle of the cone is chosen to be around the criticalangle for total internal reflection at the dense medium/air interface.That is, when light is incident at an angle normal to the axis 110 ofthe fiber 102, the light gets reflected and become grazing incident withrespect to the axis 110 of optical pipe 102. A higher half-angle of thecone helps to achieve light in a wider angular range, because light atangles less than normal with respect to the axis can also betotally-internally-reflected. FIGS. 2(a) through 2(d) illustrate a lightguide apparatus having a core 108, in accordance with variousembodiments of the present disclosure. This core layer optics traps thelight inside the core 108 and allows the light to propagatesubstantially without any loss.

Referring again to FIGS. 1(a) through 1(d), various elements as showntherein are described in further detail as follows.

In the embodiment shown in FIG. 1(a), a cylinder represents an opticalfiber or light pipe or optical pipe 102 in which light is pumped andpropagates. This structure is herein referred to as an optical pipe 102.It is to be understood that the cross-section of the cylinder A1 is notnecessarily circle. Instead, the cross-section of cylinder A1 can be,for example, an n-sided polygon (where n can be between 3 and 100), oran ellipse, or a semicircle, or bounded by two arcs of a circle.

In the embodiment shown in FIG. 1(a), the optical pipe 102 has alongitudinal axis 110.

In the embodiment shown in FIG. 1(a), a line perpendicular to thecentral axis 110 of the optical pipe 102 provides another axis. Anorthogonal co-ordinate system is used in this embodiment such that thex-axis is always oriented along the longitudinal axis 110 and y-axis isdenoted as radial axis 112. The radial plane is defined as thecross-section of the optical pipe 102 and is aligned with the y-z plane.(It should be noted that this notation is not a limitation of theseteachings.)

In the embodiment shown in FIG. 1(a), a cladding layer 104 of theoptical pipe 102 is made of a material (optically transparent medium).In one instance, the refractive index of the optically transparentmaterial in the cladding layer ranges from about 1.3 to about 1.8.

In the embodiment shown in FIG. 1(a), there is an inclusion 106 in thecladding layer 104 which may have a shape of a conical frustum. In theembodiment shown, the inclusion is an air inclusion. (It should be notedthat other optical materials can be used for the inclusion.) It isappreciated that variations on the shape of this inclusion 106 arepossible. The purpose of this cladding layer 106 is that, if any lighthits the dense-material/air interface between the air inclusion 106 andthe optically transparent medium of the cladding layer 104, the lightundergoing total-internal-reflection becomes a grazing angle (in oneembodiment, 35-0.5 degrees) with respect to the axis 110 of the opticalpipe 102.

In the embodiment shown in FIG. 1(a), there is a core 108 of the opticalpipe 102, which is further described below. This core layer 108 includesone or more optically transparent media arranged in a particulargeometry where at least one of the media in the core 108 has an index ofrefraction higher than that of the cladding material. At least one ofthe materials in the core 108 may be a lasing medium or composed of aluminescent material with light absorbing properties.

In the embodiment shown in FIGS. 1(a)-1(b), a light ray that is incidenton a side of the optical pipe 102 is incident on the optical medium-airinclusion interface at an angle higher than the critical angle for totalinternal reflection.

In the embodiment shown in FIGS. 1(a)-1(b), a light ray that isreflected from the optical medium-air inclusion interface is incident onthe core at grazing angles (in one instance, ranging from about 35degrees to about zero degrees).

In the embodiment shown in FIG. 1(b), the air inclusion is similar tothat of FIG. 1(a). The shape of the embodiment shown in FIG. 1(b) is aparabolic conical frustum. That is, the longitudinal cross-section ofthe cone looks like a section of a parabola. This structure appears tobe the most optimal results in terms of guiding the light with wideangular aperture into the core 102.

In the embodiment shown in FIG. 1(c), an air inclusion is shown that hasa shape similar to that of FIG. 1(b). The profile of the cone of FIG.1(c), as seen in the longitudinal cross-section, is a convex parabola.It is appreciated that various other shapes of FIG. 1(c) are possible,such as, for example, ellipsoidal.

In the embodiment shown in FIG. 1(d), an air inclusion 118 of conicalshape with a textures surface 122 includes one or more texturedfeatures. The shape of the textured features may be, for example, asemi-circle, quarter circle, ⅛^(th) arc of a circle, etc. The texturedsurface 122 may also be a section of a parabolic or ellipse. Also, thetop and bottom surfaces 122 and 124 can be either both concave, bothconvex, top convex/bottom concave, or top concave/bottom convex. Thesetextured surfaces 122 and 124 may also include straight lines withslopes different from each other and/or different than that of the coneitself. There may be 2-10 of these textured features on the surface ofone individual conical air inclusion 118. The texture of the uppersurface can be different from the texture of the lower surface. In oneembodiment, the textured features on surfaces 122 and 124, as a whole,may constitute an air inclusion of a spiral shape.

Further, although not shown, it is appreciated that the bottom and topsurfaces of the conical air inclusion 106, 114, 116, or 118 are notparallel, such that the top surface 122 is a cone of a higher angle,whereas the bottom surface 124 is a cone of a lower angle. The profileof the each of the cone surfaces can be any of the shapes describedherein above.

In addition, although not shown, it is appreciated that the angle of thebottom surface with respect to the central axis may be 90 degrees. Thus,the air inclusion 106, 114, 116, and 118 is bound by a circular base(whose plane is perpendicular to the surface of the central axis) and aconical surface. The conical surface can be of any shape as describedherein above.

Moreover, it is appreciated that the air inclusions need not be air orvacuum. For example, the air inclusions may be filled with opticallytransparent medium of refractive index lower than that of the materialof the cladding 104.

The ratio of the outer radius of the cladding layer 104 to the outerradius of the core 108 can be in the range of the refractive index ofthe cladding material. Thus, the ratio should be in the range of about1.3 to about 2.0.

In one or more other embodiments, the core includes a first opticallytransparent section comprising a first optical medium having a firstindex of refraction, a second optically transparent section comprising asecond optical medium having a second index of refraction, an interfacebetween the first optically transparent section and the second opticallytransparent section defining a shape, the shape, the first index ofrefraction and the second index of refraction being configured such thatlight entering the core is deflected at the interface at an angle suchthat, when the light impinges on a core-cladding interface, the lightimpinges on the core cladding interface at an angle at least equal to acritical angle for total internal reflection.

Referring to FIGS. 2(a) through 2(d), various embodiments of the core inthe system of these teachings are shown therein are described in furtherdetail as follows.

One strategy behind the optical design of the core 108 of theseteachings is to modify the angle of the light in the core 108 bycreating asymmetric interfaces (with respect the cylindrical surfaces)between two or more heterogeneous materials. Light either gets reflected(through total-internal-reflection, TIR) or refracted at theseasymmetric hetero-interfaces, such that when the light hits thecladding-core cylindrical interface again, the incident angle is morethan the critical angle for total internal reflection at thecladding-core interface. The various optical elements described below(as shown in FIGS. 2(a) through 2(d)) illustrate examples of thisapproach. One feature of this design is that the same optics has to trapthe light that enters the core 108 for the first time and then keepinteracting with the trapped light multiple times during its propagationalong the length of the optical pipe 102 without forcing it out of thecore 108.

In the embodiment shown in FIGS. 2(a) through 2(d), the core 108 thathas a cylindrical shape similar to the one described in FIGS. 1(a)-1(d).(It should be noted that this is not a limitation of these teachings.Embodiments of the cladding on the core in which the cladding of coreare not cylindrical are also within the scope of these teachings.) Thecore 108 includes at least one optically transparent medium which has arefractive index (in the range of about 1.4 to about 2.2) that is alwayshigher than that of the cladding medium and forms the boundary with thecladding which can be a cylindrical shape.

In one instance, the first optical medium 202 is a high index materialwhich can be in the shape of a cane. The second optical medium 208 is anair inclusion or a material of index different than that of the firstoptical medium 202, which can be in the shape of a cone. The refractiveindex of the second optical medium 208 is preferably less than that ofthe first optical medium 202, such that there is a possibility of totalinternal reflection at the interface between the first optical medium202 and the second optical medium 208. The half angle of the interfaceis in the range of about 0.05 degrees to about 20 degrees. The halfangle is chosen to be lower if the light incident on the core 108 fromcladding 104 is at a shallow angle. A higher angle of the interface canguide light into the core 108 even when the incidence angle of the lightis shallower. Therefore, somewhat higher angle in the range of 5-20degree is preferable.

When the angle of the conical interface is steep, the heterogeneousinterface boundary 206 is reduced to a point too quickly. Accordingly,the interface boundary can include repetition of the conical interface206 between the two heterogeneous materials along the length of theoptical pipe so that the light can keep propagating, as shown in FIG.2(c).

Referring to FIGS. 2(c) and 2(d), a second optical medium 208 interfaceswith the first optical medium 202. While the heterogeneous interface 206traps light, it still may lead to some of the light leaking out of thecore 108 and subsequently the optical pipe 102. The design shown inFIGS. 2(c) and 2(d) can achieve better performance and a substantiallylossless propagation of trapped light. As can be seen from thecross-sectional profile, the heterogeneous interface 206 between the twooptical media 202 and 208 of the core 108 has a “nested conical shape”such that one cone has a higher angle than the other. This can achievealmost completely lossless trapping and propagation of light in the core108 over lengths of optical pipe 102 greater than 1 meter.

In one exemplary embodiment, when the angle of the light incident on thecore 108 is about 20 degrees, the refractive indices of materials in thecore 108 are about 1.6 and about 1.5 (ratio=1.067), and half-angles ofthe cones, related to angle 1 and angle 2 in FIGS. 2(c) and 2(d), at theheterogeneous interface is between about 14 degree and about 26 degrees.(It should be noted that this disclosure is not limited only to theexemplary embodiment.) When the incident light angles are increased upto about 30 degree, the half-angles of the conical heterogeneousinterfaces 206 also increase proportionally while maintaining theirdifference in the range of about 12 degrees. In order for this design towork best, the refractive index difference between the two materialsmust be small (in the range of about 0.01 to about 0.2). At higherrefractive index difference, the light may start to leak out of the core108.

In one embodiment, the core has a third optical medium, which can be, inone instance, an air cavity, 210. In this design, there is a cylindrical(hollow, in one instance) space inside the core 108. The shape of theheterogeneous interface between two materials is the same as describedand shown in FIGS. 2(c) and 2(d). The ratio of radius of core 108 to theradius of cross-section of the air cavity 210 is in the range of about1.05 to about 2.0. The optimal value of this ratio is around the valueof refractive index of the core medium.

FIGS. 2(c) and 2(d) show another design variation having a first opticalmedium 212 and a second optical medium 214. The high index medium in thecore 108 can have the shape of an annular cylinder with the claddinglayer outside it. On the inner side of this annular cylinder, there canbe two optical media in the shape as shown in FIGS. 2(c) and 2(d) withthe option of having the possibility of a third optical medium 222. Thetwo heterogeneous optical media 212 and 214 have a difference ofrefractive indices in the range of about 0.02 to about 0.2. The mediumwith higher index is in contact with the high index annular cylindricalcore materials. In that embodiment, the core includes three opticallytransparent optical media with refractive index of about 1.5 to about2.2 for outermost layer, refractive index of about 1.4 to about 2.0 forintermediate layer, which has a conical interface with the innermostlayer whose refractive index is in the range of about 1.3 to about 1.9.In any circumstance, the outermost core layer has a refractive indexgreater than that of the cladding layer 104 in its vicinity by 0.1.

In one or more embodiments, the light guide apparatus of these teachingsalso includes a super-cladding layer disposed on the cladding layer,wherein the super-cladding layer includes a first optically transparentmedium that receives incident light and a second optically transparentmedium interfacing with the first optically transparent medium to definea heterogeneous interface, wherein the heterogeneous interface, thefirst optical medium and the second optical medium are configured toconvert a light beam incident on the super cladding at a first range ofangles into a light beam exiting the super cladding in a second range ofangles, the first range being wider than the second range. In oneinstance, the heterogeneous interface includes a number of bi-conicshapes.

For certain embodiments, the light guide apparatuses as shown in FIGS.1(a) through 1(d) and 2(a) through 2(d) work well without additionallayers, to guide light with an angular variation of about ±10 degreeswith respect to the normal to the central axis 110. However, many lightsources emit light with an angular spread of about ±30 degrees or more.For example, the sunlight is parallel but the sun's position has aseasonal variation of ±22.5 degrees. In such a situation, the bestalternative is to guide light prior to entry of the cladding layer andthe core. In order to pump light with an angular variation (of about ±30degrees) from the normal to the central axis 110, an additional claddinglayer (or a super-cladding layer) can be used as a light collimator.

FIG. 3 illustrates a light guide apparatus having a core, a claddinglayer, and a super-cladding layer, in accordance with one embodiment ofthe present disclosure.

A super-cladding layer 302, which includes a heterogeneous interface 310between two optically transparent media 304 and 306 in the shape isshown in the FIG. 3.

In one exemplary embodiment, the first optically transparent medium 304has refractive index in the range of about 1.3 to about 1.8.

In the exemplary embodiment, the second optically transparent medium 306has refractive index in the range of about 1.35 to about 2.0. The valueof refractive index of the first optical medium 304 is matched to therefractive index of cladding layer 104. For optimal operation, therefractive index difference between the first optical medium 304 and thesecond optical medium 306 is in the range of about 0.02 to about 0.25.

As shown in FIG. 3, in the exemplary embodiment shown therein, the shapeof the interface 310 is described as a repetitive unit whose primaryunit consists of two steep cones attached back-to-back. (It should benoted that these teachings are not limited only to the exemplaryembodiment.) The half-angle of these cones are in the range of about75-89 degrees. As can be seen from the cross-sectional view, the angle308 subtended by the surfaces of these two cones on each other at thepoint of contact is in the range of about 2-30 degrees.

The effect of the super-cladding layer 302 is to act as a “collimator”and convert light beam in a first predetermined range (in one exemplaryembodiment, about ±30 degree) into light of angle of a secondpredetermined range (in the exemplary embodiment, about ±5 degrees orless) around the normal to the axis 110 of the optical pipe 102.

FIGS. 4(a) and 4(b) illustrate ray tracing diagrams for a light guideapparatus having a semi-conical core and a semi-conical cladding layer,in accordance with one embodiment of the present disclosure. FIG. 5illustrates a ray tracing diagram for a light guide apparatus having asemi-conical core, a semi-conical cladding layer, and a semi-conicalsuper-cladding layer, in accordance with one embodiment of the presentdisclosure. FIGS. 4 and 5 show the ray tracing diagrams to illustratethe path of light pumped into the optical pipe from the sides.

In FIG. 4(a), a parallel beam of light rays 408, which are perpendicularto the longitudinal axis 110, enter the cladding layer 104 of theoptical pipe 102. The light rays 408 hit the parabolic conical airinclusions 406 at an angle above the critical angle for total internalreflection at the dense medium/air interface resulting in a reflectedlight rays at grazing incidence with respect to the longitudinal axis110 on the core 108. The rays hit the semi-conical heterogeneousinterface 414 and an angle greater that the critical angle for totalinternal reflection between optically dense medium 412 and opticallyrare medium 416, where the light rays 408 get deflected such that whenthe light rays 408 are incident on the core-cladding interface again,the angle of the incident ray is more than the critical angle at thatinterface and hence contained within the core. The trapped beam is shownas 410. In this particular ray tracing simulation, air is used for theoptically rare medium 416 (refractive index=1).

FIG. 4(b) shows the ray tracing simulation of the same system as shownin FIG. 4(a), but with various parallel beams of light rays. Each ofthese light beams lies within the same plane. This plane is at an angleof 10 degrees from the radial axis 112 which itself is normal to thelongitudinal axis 110 of the optical pipe 102. As can be seen from FIG.4(b), a majority of the light rays (410) are contained within the densemedium 412 of the core 108 via total-internal-reflection at theheterogeneous interface 414 and the core-cladding interface. Some thelight rays leak out (428) of the core since they do not meet thetotal-internal reflection criteria either at the core cladding interfaceor the heterogeneous interface 414.

FIG. 5 shows the ray traces for the optical pipe 102 with the use of asuper-cladding layer 504 on top of the cladding layer 506 and the core108. As shown in FIG. 5, a semi-conical core and a semi-cylindricalcladding layer with semi-conical inclusions. The purpose of thesuper-cladding layer 504 is to convert the input rays incident at anangle much further from the normal (with respect to the longitudinalaxis 110) to an angle closer the normal (with respect to thelongitudinal axis 110). This strategy therefore leads to a wider angularaperture of our light guide apparatus. In FIG. 5, light rays 508, 510and 512 are incident on the optical pipe 102. These light rays passthrough the super-cladding layer 504 to enter the cladding layer 506where the light rays undergo total-internal-reflection when they hit theair inclusion of the cladding layer 506 in the shape of parabolic cone.These reflected rays then enter the dense medium 516 of the core 108.Inside the core 108, the light rays undergo total internal reflection atthe heterogeneous interface 520 between dense medium 516 and rare medium518. The semi-conical shape of the heterogeneous interface changes theangle of the reflected ray with respect to the longitudinal axis 110,thus allowing the rays to meet the total-internal reflection at thecore-cladding interface. This results in light rays 514 which have beentrapped inside the dense medium 516 or the core 108.

As can be seen from the FIG. 5, the light ray 508, which is incident atan angle −22.5 degrees from the normal, gets converted to an anglecloser to the normal when it hits the cladding layer and therefore getstrapped and guided into the core 108. Similarly, the light ray 512,which is incident at an angle of +12 degrees with respect to the normalis also converted to an angle closer to the normal after going throughthe super-cladding layer 302.

FIGS. 6(a) through 6(c) illustrate ray tracing diagrams for a lightguide apparatus having a core, in accordance with various embodiments ofthe present disclosure. The embodiment shown in FIG. 6(a) is similar tothe core embodiment shown in FIG. 2(c). As shown in FIG. 6(a), lightentering the core in a predetermined range of angles with respect to anormal to the longitudinal axis is reflected in a manner that retains alight propagating inside the core.

One such ray is shown in FIG. 6(a) as ray 608 that enter the core at agrazing angle of incidence with respect to the longitudinal axis of thecore 108. The core 108 as shown in FIG. 6(a) includes two optical media606 and 604 with the heterogeneous interface 612 in the shape of nestedcones repeated along the longitudinal axis 110. As can be seen from theside view and top view of the core 108 in FIG. 6(a), the incident light608 gets trapped inside the core 108. The trapped and propagating ray610 undergoes multiple refractions and reflections at the heterogeneousinterface 612 and total internal reflection at the core surface. In thisparticular ray trace, the light ray 608 is incident at an angle of 20degrees with respect to the longitudinal axis 110. The refractive indexof optical medium 606 is 1.5. The refractive index of optical medium 604is 1.6. The half angles of the two nested cones are 14 and 26 degrees,respectively.

The embodiments shown in FIGS. 6 (b) and 6(c) are similar to the coreembodiment shown in FIGS. 2(c) and 2 (d).

FIG. 6 (b) shows the side view and top view of the core 108 as shown inFIG. 2(d). The core 108 includes three optical media 614, 616, and 618.The heterogeneous interface between 614 and 610 is in the shape ofnested cones repeated along the longitudinal axis 110. The shape of theinterface between 614 and 618 is a cylinder around a longitudinal axis110. In this particular ray tracing simulation, optical medium 618 ischosen to be air. However, the medium can have a refractive index higheror lower than medium 614 and in the range of 1.0 to 2.2. A light ray 620is incident on the core 108 at a grazing angle with respect to thelongitudinal axis 110. Three more rays are shown to be symmetricallyplaced around the circumference of the core 108, which are at the samegrazing angle with respect to longitudinal axis 110. The light ray 620after entering the core gets trapped and propagates as light ray 622after going through a series of refractions and reflections at theheterogeneous interface 619, reflections at the 614/618 boundary, andreflections at the surface of the core 108.

FIG. 6(c) shows the effect of the cladding in providing incidentradiation at angles closer to the normal to the longitudinal axis 110into the core 108 the construction of which is similar to that shown inFIG. 6(b). FIG. 6(c) shows the top view ray trace of light ray 632entering the core 108 after being reflected by a parabolic conical airinclusion 630. On entering the core 108, the light ray 632 undergoesrefracted and reflected at heterogeneous interface 619 between the twooptical media 614 and 614, reflection at the cylindrical interface of614/618 and reflections at the surface of the core. All these processestrap and propagate the light ray along the length of the core and islabeled in the figure as 634. For this particular simulation samerefractive indices and design of the core was chosen as was used in FIG.6(b).

FIG. 7 illustrates a perspective view of a solar panel including anarray of light guide apparatuses, in accordance with one embodiment ofthe present disclosure. In one embodiment, the solar panel includes anarray of parallel optical pipes, as described above, and an array ofsolar cells coupled to an end of the parallel optical pipes. In oneembodiment, the solar panel includes an optional back reflector. Thearray of parallel optical pipes shown in FIG. 7 can be side-pumped toachieve accumulated light to be collected at an end of the opticalpipes. A back reflector may be placed below the array of light pipes toensure that any light that gets lost form the light pipe can be pumpedback in again.

As shown in FIG. 7, an array of solar cells is placed at the end of theoptical pipe array. The array of light concentrators in FIG. 7 is one ofthe embodiments in FIGS. 1 (a)-1 (d). Although a back reflector is shownin FIG. 7, embodiments in which the back reflector is absent are alsowithin the scope of these teachings. An array of solar cells receivesthe light concentrated by the optical pipe array. These solar cells canbe a continuous strip or individual solar cells at the end of eachoptical pipe/fiber, and/or packaged together in serial or parallelarrangements. These solar cells can be attached to the array of lightpipes by means of an optical transparent adhesive materials, such as anepoxy resin or silicone with an index matched or closer to that of lightpipe.

If light from the sun is used as a source, the embodiment of the opticalpipe array shown in FIG. 7 brings the benefit of pumping light intoindividual light pipes even if the position of the sun changes in thesky. For example, if this array is placed such that the longitudinalaxis of the light pipe in oriented north-south direction, the variouspositions of sun going from morning to evening are equivalent in termsof the geometry of pumping the light and hence the efficiency of guidinglight into the light pipe owing to the symmetry of our optical elements.Also, as described above, the additional cladding layers can bring thefunctionality that light with wider angles of aperture normal to theaxis of the light pipe can also be guided into the light pipe. Thus,light can be pumped into the light pipe even when there is a seasonalvariation (about 45 degrees) in the position of the sun. Therefore, thelight guide apparatus of the present disclosure allows for the use ofthis optical pipe array as a non-tracking solar concentrator to collectthe light at one end of the array. This concentrated light may be usedfor various purposes. One example is for power generation by placing anarray of solar cells at the edge. It is appreciated that other uses arepossible and are described further below.

As shown in FIG. 7, an array of individual cylindrical or halfcylindrical concentrators (also referred to as optical pipes) can beassembled together, in one embodiment, to construct a light guideapparatus that can concentrate light with a wide angular aperture forincoming light. However, in other embodiments, it is also possible touse the elements of the core 108 and cladding 104 in a planar arrayssuch that the interface between core and cladding is also planar. FIG.8(a) shows such a construction, with the cladding layer 802 in a cuboidshape consisting of an array of semi-conical air inclusions 806. Eachcolumn of the semi-conical inclusions has a common longitudinal axis803. The core layer 804 as shown in FIG. 8(a) has the shape of aprismatic wedge with the angle of the wedge to be between 0.5 to 20degrees.

The top view of the cladding layer 802 in FIG. 8(b) array includes a rowand column array of semi-conical air inclusions 806. Each columnincludes an array of semi-conical inclusions that have a commonlongitudinal axis 803. The separation between each longitudinal axisequals the larger diameter of the identical conical inclusions. Thisleads to gaps 808 in the array. When light enters the cladding layer 802through the gaps 808, there is a chance that it might not hit any of theconical inclusions 806 and hence would be transmitted instead of beingguided into the core.

FIG. 8(c) shows top view of an array 810 of conical inclusions 812 whichresembles the design of fish-scale. In this case, all the properties ofindividual conical inclusions are same as that described previously.However, the conical inclusions in adjacent columns are staggered andbrought closer together such that the separation between longitudinalaxes 114 for adjacent columns is less than the larger diameter ofindividual conical inclusions. This design ensures that all the lightthat hit the cladding layer is incident on one of the conical inclusions812 and hence gets guided.

All the design variations to the conical inclusions as describedpreviously, including the use of conical heterogeneous interfacesbetween a dense medium and rare medium, holds true in the case of theplanar cladding layer as shown in FIGS. 8A-C.

While FIG. 8(a) shows the planar light concentrator using theembodiments of the light guide described in this invention, the use of acore in the shape of a prismatic wedge limits the concentration ratio(ratio of input aperture area to output area). However, as shown in FIG.8(d), a planar core 818 includes a planar array of semi-conicalheterogeneous interface 822 between optical media 818 and 820 can beused. Such a heterogeneous conical interface 822 can be obtained byusing an array of longitudinal cross-section of conical heterogeneousinterface 214 described in FIGS. 2(c) and 2(d). This conicalheterogeneous interface 822 can also be in the fish-scale design arrayas described for the conical inclusions in FIG. 8(c). Furthermore, allthe design variation to the core mentioned in previous sections,including all the optimal values of various design parameters, hold hereas well.

In one instance, the light guide apparatus of the present disclosureallows for light trapping and propagation over long distances,substantially without loss, in a planar geometry of the core describedin FIG. 8(d) and continuous pumping along the length of the structureleads to high concentration of light to be accumulated in the core. Theheterogeneous interfaces in this planar core can be a repetition of a“nested semi-cone” motif with the semi-cone angles for the two cones areabout 0.5-50 degrees and about 5-70 degrees. It is found that optimalperformance of waveguiding can be achieved in this structure, andoptical performance is achieved when the difference in angles of the twocones is in the range of about 5-20 degrees with the best value obtainedaround 12 degrees. Also, the refractive index difference between the twomaterials should be small (in the range of about 0.02 to 0.25) with bestresults obtained for a difference of about 0.07.

FIG. 8(e) shows a specific design variation such that the planar core816 has a heterogeneous interface 828 in a prismatic shape instead ofthe semi-conical shape. However, the longitudinal cross-section in theFIG. 8(d) and FIG. 8(e) is the same. In one exemplary embodiment of FIG.8(e), the two optical media 824 and 826 have the refractive index in therange of 1.3 to 2.2. However, in other embodiments, the requirement isthat the refractive index difference between the two optical media is inthe range of 0.01 to 0.3. The angles from horizontal of the two faces ofthe prismatic heterogeneous interface 828 are, in one exemplaryembodiment, in the range of 0.5 to 70 degrees and 5 to 90 degreesrespectively. The requirement, in other embodiments, is that the twoangles are close to each other and their difference should be in therange of 1 to 40 degrees. In one instance, the difference in the anglesis in the range of 4 to 20 degrees is preferred.

FIG. 8(f) shows the ray tracing simulation of the planar core 816 with aheterogeneous interface 828 as described in FIG. 8(e). As can be seen,the light beam 830 propagates through the planar core 816 without anyloss of light from the core. For this particular ray tracing simulation,it is possible to obtain a substantially lossless propagation over alength of planar core greater than 1 meter. The input light is incidenton the core 818 at an angle of 20 degrees from the cladding layer. Theangles of the two faces of the prism are 14 and 26 degrees fromhorizontal. The index of optical medium 824 and 826 is 1.6 and 1.5respectively. When the angle from horizontal of the incident light onthe planar core 816 is increased, the angles of the faces of theprismatic heterogeneous interface 828 also need to be increasedproportionally to ensure that the light is trapped and propagates in thecore 816 without any loss.

FIG. 8(g) shows a specific design variation of the planar core in whichthe longitudinal cross-section looks similar to that described in FIG.8(d) and FIG. 8(e). However, the planar core 832 as shown is FIG. 8(g)has a circular cross section in the third dimension as seen in the topview. The incident ray 838, in one exemplary embodiment, enters at anangle of 70 degrees from normal on the top surface and has a radialsymmetry with respect to the circular cross-section. The input lightaperture is the top surface while the output aperture is the cylindricalshape 842. It is evident that dramatically high concentrations can beachieved if the diameter of the cylinder 842 is chosen to be very smallcompared to the diameter of the circular input aperture.

Further, it is observed that this core design works even if thestructure is prismatic as shown in the FIG. 8(f). The criteria for lighttrapping and propagation in this exemplary structure is that the angleof light incident on the core be lower than about 40 degrees (optimalresults for 20 degrees), that the angles of the prisms should be in therange of about 2-30 degrees and about 7-40 degrees, and that therefractive index difference between two materials be in the range ofabout 0.02-0.2 (0.07 difference optimal).

For the case of planar core, the concentration ratio can be in the rangeof 100 to 1000× and can go even higher if the angular spread of theincoming light is not wide. When the angle aperture of incoming light(seasonal variation) is greater, a slight global taper in the range of0.05 degree to 2 degrees can be used to trap and propagate light intothe core. In order to address the angular spread of incoming light, onecan utilize the concept of chirping where the two angles are variedsystematically to achieve optimal waveguiding. This means that angulardifference of the two nested semi-cones in the planar core is variedperiodically, in small magnitude. In one instance, for an optimal case,the difference between the angles is increased by 0.5 degrees in eachsuccessive pair of nested-semi-cones until the difference is 14 degrees.The then the difference in angles is reduced back to 12 degrees insuccessive decrements of 0.5 degrees. The same concept of chirping alsoholds for planar core with prismatic faces as described in FIGS. 8(e)and 8(f).

FIGS. 9(a) through 9(e) illustrate a planar light guide apparatuses, inaccordance with various embodiments of the present disclosure. As shownin FIG. 9, optical elements of cladding, core and supercladding areembedded in a planar material to make a solar panel. It is evident thata planar device in which light can be pumped in at a wide angularaperture and guided to the edge is easy to manufacture and can findmultiple uses. In one aspect, the present disclosure provides afabrication method of a planar light guide device that uses side pumpingof light pipes.

FIG. 9(a) shows a scheme of operation of such a planar device (901) thatguides incident light incident at multiple angles of the sun (180degrees mornings to evening variation, and 45 degree seasonalvariation). As such, the solar energy concentrator of the presentdisclosure does not need to track the sun.

This embodiment shown in FIGS. 9 (a)-9 (c) includes a cladding layerhaving an array of semi-conical inclusions. This layer is used to turnthe incoming light into a grazing incidence. The top surface of thislayer can be semi-cylindrical or flat. The material in this exemplarylayer can have a refractive index in the range of about 1.3 to 2.0, butis always lower than at least one of the materials in the core.

All the variations in the shape of the cone described previously showncan also be used here as well. Instead of using an air inclusion, onecan also use a conical interface between two optical materials with thematerial at the bottom having a lower refractive index. The indexdifference and the angle of the cone may be chosen in such a way thatthere is total internal reflection at this interface and hence an angleof light exiting this layer is a grazing angle.

In other embodiments, a whispering gallery mode circular ring resonatorscan be used for the core to replace the wedges at the bottom, as shownin FIG. 9(c). This allows for a grazing incidence light at wider rangeof angles to be guided into the core. The diameter of circular hollowspaces are much greater than that of the wavelength of light, andtypically are in the range of about 5-500 microns.

FIG. 9(a) illustrates a planar light concentrator 901 using variousembodiments of the light guide apparatus as described in thisdisclosure. This concentrator has a wide angular aperture which isrepresented by the angles 912 and angle 914 and can be used as anon-tracking solar concentrator. The angle 912 can be in the range 0 to180 degrees and its maximum value of 180 degrees would represent thevariation in the angle of the sun from morning to evening. The angle 914can be in the range of +/−45 degrees from the normal. Its typical valueof +/−22.5 degrees would represent the seasonal angular variation of thesun during the year. Any ray of light within this solid angle boundsrepresented by 912 and 914 will be guided into the planar lightconcentrator and gets propagated to its edge. 910 represent rays oflight incident within this angular aperture. 911 represents the lightthat gets guided and propagates inside the planar light concentrator901. The planar light concentrator consists of a multi-layer stack: asuper-cladding layer 908, a cladding layer 904, a core layer 906 and alight guide layer 902 which is an extension of the core layer 906. Anarray of packaged solar cells 916 arranged in series and parallelconfiguration are attached to the edge of the planar light concentrator901 and produce electrical power using the concentrated light as theinput. This assembly including 901 and 916 can be used as a solar panelwhich uses only small area of active solar cell and does not need totrack the sun.

The cladding layer 904 consists of various embodiments as describedpreviously in this invention and is similar in construction to theplanar cladding layer described in FIGS. 8(a), 8(b) and 8(c). The planarcore layer 906 has similar construction to that described in FIGS. 8(d),8(e) and 8(f). The light guide layer 902 is basically an extension ofthe dense medium of the core. This layer is optically transparent layerof glass or plastic with refractive index matching that of the densemedium of core and is higher in refractive index than the cladding layer904. The super-cladding layer 908 is a collimating layer that takeslight rays with a angular variation represented by angle 912 andconverts them the a beam if light around the normal with an angularvariation of +/−2.5 degrees. The super-cladding layer 908 includesoptical elements described in FIG. 3 and the planar array of suchoptical elements has been described in detail in FIG. 9(d) and FIG.9(e).

FIG. 9(b) is a longitudinal cross-section of the planar lightconcentrator 901 shown in FIG. 9(a). The cladding layer 904 has beenshown here as a conical heterogeneous interface between a dense opticalmedium 918 and rare optical medium 920. The incoming light from thesuper-cladding layer 908, which, in one exemplary embodiment, is +/−2.5degrees from the normal, hits this heterogeneous interface and isconverted to grazing incidence with respect to the horizontal beforeentering the light guide layer 902 and the planar core layer 906. Theplanar core layer 906 traps and propagates the light in the light guidelayer 902. The light guide layer 902 is made of glass or plastic,provided mechanical support to the concentrator, has refractive indexhigher than cladding layer 904 and a refractive index closer to thedense medium of the planar core 906.

FIG. 9(C) illustrates the use of a micro ring resonator layer 926sandwiched between cladding layer 904 on top and light guide layer 902and planar core layer 906 on the bottom. The micro ring resonator layeris to address the inefficiency of the super-cladding and cladding layerto turn incoming light ray beyond the angular range represented by angle914 into a grazing incidence with respect to the horizontal. The ringresonator consists of the annular tube of an optical medium with highrefractive index embedded in an optical medium with lower index medium(preferably air). In this configuration of the planar light concentrator901, the angles of the faces of the heterogeneous interface planar corearray 906 are reversed from positive angles to negative. Thus the pathof trapped light in this case is inverted in the −x direction.

FIG. 9(d) illustrates the design of the planar super-cladding layer 908as labeled in previous FIG. 9(a)-9(c). The purpose of the super-claddinglayer 908 is to act as a passive collimating layer that converts anylight within the angular variation described by angle 914 into a narrowrange of angles (preferably +/−2.5 degrees) around the vertical. Theoptical element shown here is an extension of that shown in FIG. 3. InFIG. 3, the heterogeneous interface in the super-cladding layer wasshown to have a repetitive bi-conical shape around the cladding layerand its longitudinal cross-section looks like an array of steepisosceles triangles. When we move to a planar light concentrator 901,the symmetry around the longitudinal axis is no longer critical andhence this layer can have a prismatic shape. As shown in FIG. 9(d), theperformance of the super-cladding layer can be enhanced by adding aparabolic hetero-interface right below the triangles resulting in theshape of an “oil drop”. The optical medium 934 on top of the “oil drop”motif 936 is an optical medium if refractive index lower than opticalmedium 936 and is similar in refractive index to optical medium 938. Inone embodiment, a criteria for the above design of super-cladding layeris the difference in refractive index between dense optical medium 936and each of the rare optical media 934 and 938 is in the range of 0.02to 0.2. In one instance, the preferred value is around 0.07 to 0.1. Theisosceles triangle needs to be steep such that, in one exemplaryembodiment, the vertex angle is in the range 1 to 25 degrees with theoptimal value around 5 to 7 degrees. The parabola right below thetriangle should be a steep parabola with the design strategy that itsfocus should be in close proximity to converging point of the light beambeing refracted by the triangular hetero-interface.

FIG. 9(e) shows a cross section of the super-cladding layer used tocollimate incident light, 950 from incoming angles of ±22.5 degrees to anear-normal (i.e. normal to the surface of the panel, 952) orientation.The output of the cladding is near-normal light, 962 with a smallangular spread (±2.5 degrees or less) for use by the planar claddinglayer and planar core optics as described in FIGS. 9(a) and 9(b).

The cladding includes four main elements; three layers of truncatedtriangular prism arrays 954, 954, 958, followed by a parabolic lensmicroarray 960, all of which are embedded inside a material of lowrefractive index, 964.

Each triangular prism layer is a periodic array, with a repeating unitcomprising one small and one large triangle, both truncated at the topto expose a horizontal surface that allows normal (or near-normal) lightto pass through undisturbed. This makes the unit a trapezoid. However,the unit is still referred herein as a triangle or truncated trianglefor simplicity. This configuration of alternating small and largetriangles is used to ensure that one repeating unit is not in the‘shadow’ of another.

Incoming light strikes the triangular faces of the prism and undergoesrefraction, moving towards the normal. The shape of the triangles in theindividual prism layer as well as the relative alignment of the layersare so chosen to only affect the path of light rays with a large angulardeviation from the normal, while letting near-normal light passundisturbed.

There are three key requirements for the performance of the claddinglayer:

(a) There is an offset between the repeating elements of one triangularprism layer and the next. In the absence of this offset, incoming lightrays are not collimated monotonically towards the normal orientation,but instead alternate between moving towards and away from a normalorientation.

(b) Each triangular prism layer is scaled up to be twice as large as thelayer above it. In the absence of this up-scaling, initially-normallight rays are deflected away from the normal.

(c) A small difference in the refractive indices between the materialsmaking up the triangular prisms and the surrounding medium. In oneparticular instance, there refractive indices are 1.552, 1.563, 1.571,and 1.583 for the materials making up 954, 954, 958 and 964respectively. While different materials can be used to make thedifferent triangular prism layers, it is not a requirement and all thelayers could be made out of the same two material set. The only keyfactor is their refractive index difference is small.

The three triangular layers are followed by a parabolic concave lensmicroarray that further “planarizes” the light to a more normalorientation. The lens faces 966 and 968 are nominally parabolic in thisconstruction, although other surface shapes are also equally valid. Thisparabolic layer is used to collimate the light beam even further and issimilar to the parabolic heterointerface described in FIG. 9(d).

FIG. 9(f) shows data for one embodiment of the structure in FIG. 9(e).FIG. 9(f) shows angular range of the output rays for one embodiment ofthe structure in FIG. 9(e) when the input angular range is +/−22.5degrees form normal. As can be seen from the FIG. 9(f), the angulardivergence of the output beam is +/−22.5 degrees. In this particularsimulation, we found that the 97.5% of the input light in the angularrange of +/−22.5 degrees is collimated into an output beam of angularrange +/−2.5 degrees.

The materials used to fabricate the light guide apparatus as describedin this invention can be glass, carbon based polymers, plastics, smallorganic molecules, polymers based on silicon-atom-backbone such assilicones and siloxanes. One may also choose to use mixtures ofsiloxanes with other polymers and small molecules to achieve specificmechanical properties as well as optical properties such as refractiveindex and dispersion. As described hereinbelow, these materials may beused in the liquid form and cross-linking agents added in small amountsto achieve a solid, semi-solid, elastomeric or gel-like layer afterapplication of heat or UV light. Also, fluorinated polymers andfluorinated silicones and siloxanes may be specifically used to achievelow refractive index than the base non-fluorinated versions of the samepolymers. Similarly, a surfurized version of the polymers may be used toachieve a higher refractive index than the non-sulfurized versions ofthe same polymers. In some uses of the present disclosure, some liquidswith different refractive indices such as water, mineral oil, organicsolvents, fluorinated liquids etc may be also used. Also, for variousembodiments described in this disclosure, we may use glass of variousrefractive index (1.4 to 2.2). In some instances we may use glass thatmay have low melting temperature to enable molding of glass.

Refraction at a polymer (or glass)/air interface can lead to splittingof the white light due to differences in the refractive index of thedense medium at various wavelengths (dispersion relation) while theindex of air remains constant. However, if refraction happens at theinterface of two optically dense materials with different refractiveindices (n1 and n2) such that the ratio of n1/n2 is constant for eachwavelength, then light of each wavelength is refracted by the samemagnitude and hence no chromatic aberration. In optical simulations ofthe present disclosure, the choice of materials such that the materialof higher index has a higher dispersion (low abbe number) while amaterial of lower index has a lower dispersion (higher abbe number) ismade. This strategy is also leads to a much wider choice of availablematerials since many of the higher index materials have low abbenumbers. FIG. 10(a) shows the prescriptions for Materials Selection inthe light guide apparatus of the present disclosure.

Materials with engineered refractive index can also be used by selectivedoping the optical media with small quantities (0.01% to 0.00001%) ofdyes and chromophores that absorb light in specific parts of thespectrum. This strategy creates a tailored refractive index vs.wavelength distribution for one or both materials which can lead towave-guiding for only certain parts of the spectrum. Materials whoserefractive index vs. wavelength distribution has been engineered bymixing two materials with different refractive index vs. wavelengthdistributions can also be chosen. FIG. 10(b) shows an example ofwave-guiding using above described strategy of material selection. Herewe have chose to set a “cutoff” by choosing a materials combination forthe cladding layer such that the n₁/n₂ decreases slowly with wavelengthgoing from 400 nm to 800 nm wavelength. At 800 nm, the n₁/n₂ ratio fallsblow the threshold that is required for the incident light to undergototal-internal-reflection at the heterogeneous interface. Therefore the800 nm light (and beyond) gets transmitted through the cladding layerwith slight deviation in its path and hence does not contribute to thewave-guiding process. FIG. 10C shows the data for wave-guiding andtransmission of light as a function of wavelength of light forpredetermined material combination in the cladding layer. As can be seenfrom the figure, 100% of the light beyond 800 nm gets transmitted (nowave-guiding at all) while there is clear wave-guiding over 1 m lengthand ˜60% collection of light at the edge for light 400-800 nm range.

A similar strategy can be used in the core layer such that the n₁/n₂ratio is different for the materials in the core layer and this wouldresult in leakage of selected wavelengths of light from the core andwave-guiding of rest of the wavelengths of light in the spectrum.

The light guide apparatus as described in this invention can be made bywidely used molding process. A mold which is negative of the design ofthe part that needs to be made. A cross-linkable liquid polymer or gelcan be poured into the mold them peeled off once it solidifies. The moldcan also be in the shape of a cylindrical drum, the rotating action ofwhich can be used to prepare the parts in a continuous mold-and-peel,high speed roll-to-roll process.

In one embodiment, the present disclosure provides a method forfabricating the cladding layer or core in the light guide apparatusdescribed above. As shown in FIG. 11(a), a first optically transparentmaterial has a surface having a male portion 1102 (a plurality ofprotrusions) formed thereon. The first optical transparent material canbe engaged with a second optically transparent material. The secondoptically transparent material has a surface having a female portion1104 (a plurality of recesses or indentations) formed thereon. In oneembodiment, the male and female portions are configured to have similarshapes, and the female portion is configured to be bigger than the maleportion to leave a space for air inclusions 1106 in between afterassembling them together. A location of each protrusion corresponds to alocation of each indentation, thereby forming a number of inclusions.This fabrication method can be used to produce any light guide assemblythat contain air inclusions, such as the core 108 and the cladding layer104 as shown in FIG. 1(a).

In an exemplary embodiment, for the cladding layer, the male portionwould have semi-conical protrusions on its surface and female portionwill have semi-conical recesses on the surface. The recesses, in thatexemplary embodiment, are slightly bigger in size (5-50 microns) in eachdimension. When both the parts are assembled together to make one piece,there are air inclusions trapped inside the finished piece whosedimensions are equal to the difference in the dimensions of protrusionsand recesses.

In one embodiment, the method further includes depositing a layer of athird optically transparent material over the surface of the firstoptically transparent material. The layer of the third opticallytransparent material may have a substantially constant or varyingthickness, which is configured such that a shape of the surface of thefirst optically transparent material, after deposition, is substantiallycongruent with a shape of the surface of the second opticallytransparent material. After assembling, the third optically transparentmaterial is disposed in the space between protrusions and indentations.

In one embodiment, the male portion 1102 has semi-conical protrusions onits surface, and the female portion 1104 has semi-conical recesses onits surface. The recesses are slightly bigger in size (5-50 microns) ineach dimension. When the first and second optically transparentmaterials are assembled together to make one piece, there are airinclusions trapped inside the finished piece whose dimensions are equalto the difference in the dimensions of protrusions and recesses.

To have a complete cone assembly, two such parts with semi-cones can beassembled together to form part with full cone inclusions. In order tohave air inclusions in finished part with special geometry, such asspiral air inclusions, the conical protrusions and recesses can havesurface texture. The protrusions have concave surface texture while therecesses have convex surface texture. If the cross-sections of thetexture is desired to be a full circle, each of the concave and convexsurface texture may be a semicircle. Further, it is also possible tohave convex and concave surface textures which are arcs of a circle sucha 90 degree arc, 45 degree arc, and so on.

In another instance, where the inclusions are not air inclusions but areinclusions of a third optically transparent material different from thefirst and second optically transparent materials, a layer of the thirdoptically transparent material of predetermined thickness is depositedon the male portion. The predetermined thickness is equal to about thedifference in size between the protrusions in male portion and therecesses in the female portion. When the two parts are assembledtogether, the inclusions of the third optically transparent material aredisposed between the male and female portions.

The first and second optically transparent materials and the size of theinclusions are selected such that a predetermined deflection of light,incident on the assembly within the range of angles with respect to anormal to a longitudinal axis, is obtained.

In yet another embodiments, the embodiment includes two differentmaterials with a specific shape of the boundary of the two materials(heterogeneous interface). In fabricating such an embodiment, the firstoptical layer is prepared using molding method described above using amold that has the shape of the heterogeneous interface to be prepared.The molded layer is then filled with the second optical material tocreate a final layer consisting of two different optical materials witha heterogeneous interface in a predetermined shape.

Although the light guide apparatus of the present disclosure has beendescribed as a sun light concentrator, it is appreciated that other usesof the present disclosure are possible.

The planar light collector in various versions of FIG. 9(a)-9(e) usesvarious layers of optically transparent materials with the heterogeneousinterface between to materials in a predetermined geometry. While it isassumed that the layers in this light guide apparatus be solid tomaintain mechanical integrity, one or more of the optically transparentmaterials may be liquid. These liquid layers may be sandwiched betweensolid layers, physically encapsulated to prevent their leakage andperform same optical functions as the solid materials of the samerefractive index.

Also, in some manifestations of this invention, there can be airinclusions. Injection and withdrawal of optical fluids from the airinclusions in the cladding and the core cavity can result in loss ofoptical wave-guiding since the total internal reflection needs aninterface between dense and rare medium. This concept can be used in ourdesign to make smart windows which produce power when waveguiding is on(no liquid in cavity) and act as transparent windows when waveguiding isoff (cavities filled with liquid).

In a manifestation of this invention where an optical liquid layer isused as a lower refractive index, the TIR happens at the interface ofthe dense solid medium and the rare liquid medium. If this liquid ispumped out of the device and replaced by another liquid whose refractiveindex is equal to the dense medium, the TIR at the interface of twomaterials is prevented and light is transmitted instead of being guided.Therefore a power producing smart window with an active control of lighttransmission (and guiding) of light using can be demonstrated using theinflux and out-flux of optical fluids in our light guide apparatus.

FIG. 12(a) illustrates the concept of the smart window using the lightguide apparatus as described in this invention.

Using the principle of reversibility of light, the light guide apparatusof the present disclosure can be used as a lighting device. A source oflight is placed on one of the edges of the light guide apparatus suchthat the light emitted by the light source gets guided into the core andprogressively leaks out of the light guide apparatus. In this case, theoutput of the light guide apparatus is its top surface that providesuniform illumination with a wide angular divergence of the output lightrays. FIG. 12(b) shows an array of light emitting diodes acting as lightsources place on the edge of the light guide apparatus.

The planar solar concentrators as described in FIG. 9(a) can be used forindoor lighting applications by replacing the solar cells on the edge ofthe light guide apparatus by an array of optical fibers. Theconcentrated light that is collected at the edge of the light guideapparatus can be pumped into individual optical fibers coupled to theedge. These optical fibers can then taken to the lighting fixturesinside a building to illuminate the interiors with natural day lighting.

Solar Thermal: The planar optical concentrators can be used to guideconcentrated light onto and evacuated tube solar thermal water heater.The concentrated light makes the tube much hotter than in the regularwater heaters. This will make the water heater more efficient because ofthe higher thermodynamic efficiency at a higher temperature of the tube.This scheme can also be used for solar thermal application where athermal fluid flows inside a tube which runs through the edge of acascade of planar concentrator. As an example, our calculation show thata cascade of 10 panels of 1 m×1 m area in this scheme can heat thethermal fluid at a temperature to 400 degrees if the mass flow rate ofthe thermal fluid is kept as a 0.06 kg/s through the tube.

FIG. 12(e) shows the schematic of the light trapping optics that we havedemonstrated in our light guide apparatus to be used on top of aphotovoltaic device to trap the light that leaves the photovoltaicdevice without getting absorbed. Such light trapping optics relies onthe total internal reflection at the heterogeneous interface of the twooptical materials. The optical laminate layer redirects the light backonto the photovoltaic device by converting the angle of the high anglephotons to an angle that lies within the total internal reflection coneof the dense material/air interface.

One of the problems of luminescent concentrators is that the luminescentmaterial emits light at all angles but only the emitted light that liewithin the TIR cones is contained within the slab. The light emittedbeyond the TIR cone is lost resulting in lower performance of theluminescent concentrators. These TIR losses will be overcome bylaminating a multilayered stack of engineered optical layers whichmodify the angle of the light emitted beyond the TIR cone to shallowerangles using the principles used in the design of the core in the lightguide apparatus described in this invention. The design of opticallayers as shown in FIG. 12F, consists of two optical polymers with arefractive index difference <0.1 and an interfacial structure consistingof an array scalene triangular prism with an included angle in the rangeof 10°-15°. The angles of the scalene triangles are chosen to achievethe TIR at the interface of the two polymers and trap the light emittedbeyond the regular TIR cone. The small included angle and low refractiveindex difference is effective in propagating the trapped light withminimal loss.

FIG. 12(g) illustrates the use of the concentrated light output of lightguide apparatus as described in this invention as an input for theoptical pumping of the lasing medium in a laser device. The input to thelight guide can be sun light or an array of diode lasers incident on thetop surface of the planar optical concentrator. Various schemes havebeen proposed in the art to achieve optical pumping of the lasing mediumbeyond its lasing threshold. A high power source in a compact geometryis required to optically pump the laser. The concentrated light outputat the edge of the light guide apparatus can be an effective method toachieve this goal as shown in FIG. 12(g).

For the purposes of describing and defining the present teachings, it isnoted that the term “substantially” is utilized herein to represent theinherent degree of uncertainty that may be attributed to anyquantitative comparison, value, measurement, or other representation.The term “substantially” is also utilized herein to represent the degreeby which a quantitative representation may vary from a stated referencewithout resulting in a change in the basic function of the subjectmatter at issue.

Although embodiments of the present disclosure has been provided indetail, it is to be understood that the method and the apparatus of thepresent disclosure are provided for exemplary and illustrative purposesonly. Various changes and/or modifications may be made by those skilledin the relevant art without departing from the spirit and scope of thepresent disclosure as defined in the appended claims.

What is claimed is:
 1. A light guide apparatus comprising: a coredefining a longitudinal axis; and a cladding layer on the core, thecladding layer comprising a first optical medium having a first index ofrefraction and an inclusion structure embedded in the first opticalmedium; the inclusion structure comprising a second optical mediumhaving a second index of refraction; the inclusion structure defining aninterface between the first optical medium and the second opticalmedium; the inclusion structure, the first index of refraction and thesecond index of refraction configured such that light incident on theinterface is deflected, deflection comprising refraction, such thatlight impinging on an interface between the first optical medium and anexterior medium impinges at an angle at least equal to a critical anglefor total internal reflection, and propagates at a predetermined grazingangle with respect to the longitudinal axis; the light being incident ina predetermined range of angles from a normal direction substantiallyperpendicular to the longitudinal axis.
 2. The light guide apparatus ofclaim 1, wherein the interface has one of a conic shape, a semi-conicshape, a parabolic conic shape, and an ellipsoidal shape.
 3. The lightguide apparatus of claim 1, wherein surfaces of the inclusion structureare textured.
 4. The light guide apparatus of claim 1, wherein thesecond optical medium is air.
 5. The light guide apparatus of claim 1,wherein a cross section of the cladding layer has an outer circumferenceof a shape selected from the group consisting of a circle, an N-sidedpolygon, an ellipse, a semicircle, and a bounded shape of two circulararcs.
 6. The light guide apparatus of claim 5, wherein N is a naturalnumber ranging from 3 to
 100. 7. The light guide apparatus of claim 1,wherein the inclusion structure has a semi-conic shape, and the core hasa semi-cylindrical shape.
 8. The light guide apparatus of claim 7,wherein the core is tapered.
 9. The light guide apparatus of claim 1,wherein the first refractive index of the first optical medium of thecladding layer ranges from about 1.3 to about 1.8.
 10. The light guideapparatus of claim 1, wherein the core comprises at least an opticallytransparent medium, and a refractive index of the optically transparentmedium of the I core is greater than that of the first optical medium ofthe cladding layer.
 11. The light guide apparatus of claim 1, whereinthe core comprises: a first optically transparent section comprising athird optical medium having a third index of refraction; and a secondoptically transparent section comprising a fourth optical medium havinga fourth index of refraction; an interface between the third opticallytransparent section and the optically fourth transparent sectiondefining a shape; the shape, the third index of refraction and thefourth index of refraction being configured such that light entering thecore is deflected at the interface at an angle such that, when the lightimpinges on a core-cladding interface, the light impinges on thecore-cladding interface at an angle at least equal to a critical anglefor total internal reflection.
 12. The light guide apparatus of claim11, wherein the shape comprises a first conical frustum of a first halfangle; and wherein a central axis of the first conical frustumsubstantially coincides with the longitudinal axis of the core.
 13. Thelight guide apparatus of claim 12, wherein the core further comprises athird transparent section comprising a third optical medium, the thirdtransparent section interfacing with the second transparent section todefine a central cylinder.
 14. The light guide apparatus of claim 13,wherein the shape further comprises a second conical frustum of a secondhalf angle, a central axis of the second conical frustum substantiallycoinciding with the longitudinal axis of the core.